Image development



Marh 24, 1959 E, H, LEHMANN 2,879,397

' IMAGE DEVELOPMENT vFiled DeG. 19, 1955 United States Patent O IMAGEDEVELOPMENT Ernest Henry Lehmann, Rochester, N. assigner to Haloid XeroxInc., a corporation of New York Application December 19, 1955, SerialNo. 554,068

6 Claims. (Cl. Z50-65) This invention relates in general to X-rayrecording and in particular to a new improvement in X-ray recordig byxerography.

The art of X-ray recording by xerography, generally known asxeroradiography, relates to the recording of X-ray patterns andinformation by means of materials and devices whose electricalconductivity is altered by the action of penetrating radiation, such asX-rays and the like. It has been found that a normally insulating X-raysensitive layer overlying a conductive backing surface may be chargedelectrically and used as a radiation sensitive recording surface becauseof conductivity imparted by the radiation. Thus, the appropriatesurface, such as for example a metallic surface having a vitreous oramorphous selenium layer, may be charged and exposed to an X-raypattern, as disclosed in Schaifert et al. 2,666,144. The result of thischarging and exposure is an electrostatic latent image orxeroradiographic latent image which may be developed by dusting withfinely divided charged particulate material to form a visible image.

In present operations, according to the art of xeroradiography, it isusual to charge and expose an appropriate xeroradiographic plate, afterwhich the image is developed by presenting to the plate surface a cloudor gas suspension of charged powder particles. The image as developed inthis way is characterized by a property known in the art as halo, inwhich discontinuities in the charge or potential of the xeroradiographiclatent image are accentuated in the developement process. Thus, areashaving a high charge are developed by the deposition thereon of amoderately larger amount of powder, and areas having a lower charge aredeveloped by a moderately lighter powder deposition, while in boundariesbetween two such areas, and particularly in relatively sharp areasbetween the highly charged and lower charged areas, there is an absenceof powder deposition resembling very much a light or clear halosurrounding the dark area. Similarly, the heaviest deposits are notnecessarily in the areas of highest potential but are, instead, justinside the borders of highly charged areas. This property has in thepast been of considerable value in X-ray recording, since it accentuatesdiscontinuities or flaws in relatively uniform test objects. As a resultof this, the contrast sensitivity of the xeroradiographic image issubstantially increased and, in addition, the exposure latitude isincreased, inasmuch as discontinuities or llaws can be detected both inthe lighter and the darker areas of the image.

Unfortunately, the property of halo, which is the very property thatproduces these preferential results, produces also an additional anddetrimental result. Very frequently in X-ray examination it is desiredin industrial castings to locate casting aws, or in medical examinationto locate and analyze tissue and bone areas when the area of particularinterest in the X-ray examination is an area at or near a point of sharpcontrast differential. Thus, in medical examination, it may be desiredto detect a bone injury at or near a joint or, in industrialexamination, it may be desirable to detect tlaws which might be ricemore prevalent near seams, joints or sharp contour changes in the testobject. For example, if a casting is being analyzed by X-rayexamination, the purpose of the examination very often is to detect thepresent or absence of aws associated with sharp angles in the surfaces.lt is exactly this type of tlaw that could occur in the halo area of thexeroradiographic plate and which is somewhat more likely to be partiallymasked by the halo itself.

It is, therefore, an object of the present invention to provide newmeans, methods and apparatus for xeroradiographic examination whereinone anticipated advantage is increased readability of the X-ray recordin areas of sharply varying contrast.

It is another object of the invention to provide new means, methods andapparatus for xeroradiographic examination in which there is enhancedcontrast sensitivity with particular reference to contrast sensitivityin areas of sharp contrast gradation.

Additional objects of the invention will in part be obvious and will inpart become apparent from the following specification and drawings inwhich:

Fig. 1 is a perspective view of an illustrative test object.

Fig. 2 is a side cross section of the test object of Fig. 1.

Fig. 3 is a diagrammatic presentation of a xeroradiographic print of thetest object of Figs. l and 2 produced according to prior art methods.

Fig. 4 is a diagrammatic presentation of a xeroradiographic printproduced according to the methods of the present invention.

Fig. 5 is an enlarged fragmentary view of a portion of axeroradiographic print as illustrated in Fig. 3.

Fig. 6 is an enlarged fragmentary view of a portion of axeroradiographic print as illustrated in Fig. 4.

Fig. 7 is a diagrammatic illustration of a probable mechanism ofelectrical operation according to the present invention.

Essentially, the operation of the present invention calls for a dualdevelopment procedure in which a xeroradiographic latent image is rstdeveloped under conditions enhancing detail presentation in certainareas of the image, and the image is subsequently again developed underdifferent conditions of opposite charge, enhancing detail presentationin the remaining areas of the image. Preferably this is accomplished bysuccessive development steps in one of which development is carried outwith a cloud or gas suspension of finely divided particles of one colorand electrical polarity followed by a second development step with acloud or gas suspension of particles of a contrasting color and oppositeelectrical polarity. As a result of this dual operation, particles ofthe one color are preferentially deposited or developed in relativelylarge areas and areas of gradual contrast differential, whereasparticles of the contrasting color are preferentially developed ordeposited in what are normally the halo areas of a xeroradiographicprint. By these successive contrasting development steps there isproduced a xeroradiographic print retaining essentially the advantagesof enhanced readability produced by the halo characteristic while at thesame time substantially overcoming the masking effect of halo withrespect to sharp contrast areas.

For the manufacture of developer materials of appropriate colors andtriboelectric properties, a suitable dye or pigment may be added to ausable resin carrier or binder, and the colored resin may then bepulverized to form a fine powder, preferably of about 5 micron particlesize. Desirably for many purposes it is ultimately useful to fuse ormelt the developer to form a permanent image, although for many testpurposes a fusible image is not necessary. In the manufacture of athermoplastic toner or developer, a suitable resin such as aresin-modified phenolformaldehyde resin, an acrylic type resin, apolystyrene resin or the like may be mixed with a dye or coloringmaterial such as malachite green oxalate, Victoria Pure Blue B. O.,Methylene Blue Chloride and Phthalocyanine Blue, and pulverized to formthe desired powder. Developer compositions. that have been found todisperse into a positively charged cloud, and thus accomplish reversaldevelopment include,` for example, Du Pont Oil Red and a medium high.molecular weight polystyrene resin having a ball and ring melting pointof 125 C., Quinoline Yellow with the same polystyrene and with polybutylmethacrylate, and Oil Blue ZN with polystyrene and polybutylmethacrylate. Oppositely, or negatively charged developers for directdevelopment include, fcr example, Victoria Pure Blue B. O. and the samepolystyrene resin Methylene Blue Chloride and polystyrene, andPhthalocyanine Blue and a rosin modiiied phenol formaldehyde resinavailable under the name Amberol F-7l. In addition, other colored powdermaterials may be selected from existing and available powders, anddepending on the polarity of their charge when dispersed into a cloud,may be used for positive or negative deposition.

Example I A test object was examined by xeroradiography according to thefollowing procedure. The X-ray sensitive recording plate was a flatmetal plate having an 8O micron layer of vitreous selenium on itssurface and available from The Haloid Company of Rochester, New York.The plate was sensitized by charging its surface uniformly to apotential of about 600 volts and a casting to be examined was placedbetween the plate and the X-ray source. The casting had a thicknessranging between 1A inch minimum and 2 inches maximum and exposure to a100 k.v.p. X-ray source was set at milliamperes for l0 seconds at 30inches target-object distance. The result of the charging and exposurewas a developable xeroradiographic latent image.

The latent image as produced was developed iirst with a bluish-grayappearing powder such as one containing malachite green oxalate in apolystyrene resin. This material was sprayed into a powder cloud througha steel nozzle and the cloud was thus charged to predominantly negativeelectrical polarity.V The suspension was then presented to the invertedsurface of the image plate whereby the particles were depositedpredominantly in the relatively more highly charged areas of thexeroradiographic image. Because the image surface was held upside down,gravitational deposition was avoided. The image was then given a seconddevelopment operation in which it was developed with a reddish coloredpowder such as a mixture of a polystyrene resin with a dye availablefrom the Du Pont Company of Wilmington, Delaware, under the name Du PontOil Red. The red powder was sprayed through a steel nozzle and thusreceived a positive polarity electric charge so that it depositedsubstantially in the so-called halo areas of the xeroradiographic image.As thus deposited, the blue-gray powder produced the primaryxeroradiographicimage and the contrasting red colored powder effectivelyfilled in the halo areas to produce a xeroradiographic print ofsubstantially improved readability. The developedimage was examined onthe xeroradiographic plate itself and was transferred by pressurecontact to a coated paper surface.

Example II The procedure of Example l was repeated in variouscombinations with positively charged powders and negatively chargedpowders as indicated below.

In diiierent procedures the image was first developed with thenegatively lcharged powder followed by subsequent development with thepositively charged powder and in reverse operation, iirst with thepositively charged and next with the negatively charged powder. Care wastaken to employ contrasting colors for the different developmentoperations. The developed images were characterized by betterpresentation of detail n the halo areas. In particular, where the areaof primary interest in the examination was in the relatively thinnerparts of the casting, development with the positively charged powder wascarried out rst and development with the negatively charged powder inthe subsequent operation. ln the event, however, that generalexamination was desired or in the event that detail was consideredprimarily important in the thicker areas of the casting, the iirstdevelopment operation was carried out with the negatively chargedpowder. The powders employed were various of the powders such asillustratively listed hereinbefore.

It is believed that a better understanding of the present invention ispossible in conjunction with the iigures set forth in the drawings. InFig, 1 there is illustrated a casting generally designated 10 having abroad or relatively thinner area 11 and thicker areas 12 such as acircumferential ring or the like. Illustrated in the iigure is a flaw i3in the casting extending into both the thinner and thicker areas.

In Fig. 2 is presented a cross section view of the same castingillustrating the comparative differences in thickness.

In Fig. 3 is illustrated diagrammatically a developed image according toprior xeroradiographic techniques in which the xeroradiographic plate ischarged to positive polarity and exposed to a test pattern ofpenetrating radiation such as, for example, a pattern of radiationresulting from the passing of X-rays from an X-ray source through thetest object of Figs. l and 2 and onto the xeroradiographic plate.Following exposure in this manner the xeroradiographic plate isdeveloped by deposition thereon of negatively charged powder, as inaccordance with the first portion of Example l, to yield axeroradiographic print generally corresponding to the print illustratedin Fig. 3. The print, generally designated 10a, shows a relativelyheavier image deposit 12cz in the thicker or ring area 32a correspondingto the thicker portion of the casting and a relatively lighter depositin the inner portion 11a corresponding to the thinner portion of thecasting. The crack i3 of the casting is reproduced as a aw line 13a inthe xeroradiographic print. Immediately on the thick side of theboundary between the thick and thin portions of the casting is a heavyhalo area 14 of excessive developer deposition and immediately insidethe boundary is a light halo area i5 of substantially no developerdeposition. r[he flaw line 13a or other evidence of casting defect islargely masked by the overriding elect of the halo, particularly in thelight halo area.

ln Fig. 4 is illustrated diagrammatically a xeroradiographic printproduced by the complete procedure of Example I. Illustrated in thisfigure is the thick image 12b corresponding to the thicker portion ofthev casting and the thin image lib corresponding to the thinnerportion. Between these two areas is the halo area image portion 15bcorresponding to the second color development of Example l. In thisportion of the halo area, the flaw line 13b is visible, and this fact iscontrasted to the comparable portion of the image in Fig. 3.

In Figs. 5 and 6 is illustrated in greater detail the deciency accordingto the prior art and the elimination of the deficiency according to thepresent invention. In Fig. 5 is illustrated an enlarged portion of theprint of Fig. 3 showing the outer ring area 12a and the inner area 11awith the halo area 15 between the two larger areas. lt is also notedthat a very dark image area 14 surrounds the outer ring and is in factpositioned tol correspond to the edge of dark areas and thus tocorrespond to areas of greatest gradation of electric potential in thexeroradiograpbic latent image. Directly adjacent to the dark area. 14 isa corresponding light or halo area such as halo area l5 between the ringand the central area or such as the non-image area immediately outsidethe image. Within this halo area l5 there is substantially no detailvisible in the xeroradiographic print. It is observed with referenceback vto Fig. 3 that the flaw line 13a which is visible through the darkarea 14 disappears substantially completely as it passes through thehalo area.

In Fig. 6 is illustrated a comparably enlarged portion of the modifiedXeroradiographic print illustrated in Fig. 4. Here the dark or edgelines 14 are retained and remain in predominately the same depositioncolor as area 12a. In a preferred example the original deposition toform area 12a was a greenish-gray image powder and the subsequentdeposition was a red powder. In the comparable area 12b the compositeimage was predominately a gray-green with only a faint trace of red.Generally similar in appearance was the inner area 11a of the firstcolor development corresponding to the thin section of the test object.This area largely retained its original or in the preferred embodimentgray-green tone with, however, a somewhat more prominent red under-tone.Between edge line 14 and the inner area 11b corresponding to theoriginal halo area 15 is an area 15b containing a substantial deposit ofthe second image color. In the preferred embodiment this was a clearlyapparent red Yimage area showing full detail of casting structure fromthe test object. This halo area was almost wholly the second imagecolor, or in the preferred embodiment a red image, and substantiallyfree of the first image color. It

extended in gradually increasing density from the rather low printdensity of the second color in the inner area 11b to a heavy printdensity in the halo area 15b. At its -other edge, it extendedsubstantially up to the dark area 14. There was a very fine line ofdemarcation between the edge of the red deposit of the halo area and thedark area, but even in this sharp area of demarcation there was atendency of the second developing color to spill over into the boundaryarea between them so that no image area in this entire zone was entirelyfree from deposited developer material. Referring back to Fig. 4, itisobserved that the flaw line 15b is visible through the original haloarea even to the extent of being visible in the boundary area betweenthe filled-in halo and the dark area.

As a result of the present invention, xeroradiography including themethods of the present invention is capable -of xeroradiograpnicanalysis for aw detection with contrast sensitivity at least as fine as1% contrast sensitivity. Even in halo areas excellent contrastsensitivity is observed.

In Fig. 7 is illustrated one explanation of the operation of the presentinvention, although it is to be understood that this explanation ispresented in illustration of the invention and not in limitation. InFig. there is lshown diagrammatically a xeroradiographic print generallydesignated 21 comprising a conductive backing member 22 and an X-raysensitive insulating layer 23 disposed on the surface of the conductivebacking. Illustrated in the figure are two thick area images 25corresponding to the thicker areas of the test object and a thin areaimage 26 corresponding to the thinner areas of the test object. Asillustrated by the plus marks in the ligure, the relatively thicker testobject areas correspond to a higher potential or more dense electriccharge remaining on the plate as the xeroradiographic image, whereas thethinner areas of the test object are represented by a relatively lowercharge as illustrated by more widely-spaced plus marks in the drawing.The lines of force associated with the charged image are illustrated asgenerally emanating from the image surface with, however, substantialdistortion of the field of force being caused by fringing at the areasof highest charge. Thus, at the boundary of the highly charged areas,relatively larger proportions of the lines of force associated with thedense charge fringe outwardly and then pass back into the image surfaceto yenter the conductive backing surface. Thus, immediately adjacent tothe areas of highest charge there is an effective reverse field of forcecaused by this fringing of the lines vof force of the image. The resultis that negatively charged powder particles will be attracted to boththe highly charged image areas and the moderately charged image areas,but will be actively repelled from the halo area surrounding the highlycharged portions. This property is utilized in the halo area developmentoperation by the deposition of positively charged powder particles whichare at`n`rmatively attracted to these halo areas. As 1s apparent, thedeposition in the halo areas is an image deposition in that it clearlyreects image characteristics of the charge pattern.

The two-color images prepared according to the present invention arecharacterized in that they present a greater amount of analytical detailin areas of widest charge differential in the xeroradiographic latentimage. In particular, flaws to be detected in areas adjacent to widecontrast ranges are more clearly presented in the resultingxeroradiographic print.

In achieving the results of the present invention, it is particularlyimportant to employ in the successive development steps two developmentmaterials of contrasting color as well as of contrasting chargepolarity. If, for example, materials of relatively similar color densityand hue are employed, the contrast in the halo areas is destroyed andthe image becomes muddy in appearance and grossly lacking in detail.Similarly, if the image material in the two development steps is not ofcontrasting polarity, then there is a failure to till in the halo areas.Likewise, if attempts are made to use the contrasting powderssimultaneously, it has been found that the advantages of the inventionare not realized.

As a general guide, the major development is carried out with developerparticles of electric charge having opposite polarity to the chargeoriginally applied to the xeroradiographic plate for sensitization. Thispolaritv may be imparted by forming the xeroradiographic devel.- operinto a cloud or mist and subsequently charging the cloud or mist to thedesired polarity by suitable means such as, for example, by passing thecloud through an area of corona discharge or through an area ofradioactive discharge. It has been found, however, that by suitableselection of the powder material and by suitable selection of capillarymaterial, a powder spray may be charged predominantly to a desiredpolarity by passing it under turbulent conditions through a tinecapillary. In this manner, positively charged powder clouds have beenformed using a wide variety of powder materials of varying colors. As ageneral guide, it is usually desirable to employ a relatively neutralcolor such as a blue, gray, green, or the like which is visible in sharpcontrast against a white background, for the primary developmentoperation so that the image on the gross basis is easily visible againstthe white background. It' desired, a white image may be employed andviewed against a dark plate surface without transfer.

The second development operation is carried out using a developermaterial of contrasting color and, as stated before, a contrastingpolarity. In general, this is designed to be readily visible in contrastto the particular powder or developer' material chosen for the primarydevelopment. This material, similarly, may be empirically selected byexperimentation with turbulent passage through a fine capillary toselect those materials which charge to opposite polarity. Thus, when thexeroradiographic plate is sensitized to positive polarity, the secondarydeveloper will be selected to be of positive electric charge. Ifdesired, a material of either positive or negative frictional chargingmay be formed into a cloud and charged to the desired opposite polarityby means of corona discharge or the like. This developing material is ofcontrasting color, and usually of what is best described as a colorfulshade such as red, rose, bright blue, bright green, lavender, or thelike, to be clearly contrasting with the other developer.

As a preferred embodiment of the invention, it has been found thatdevelopment of the secondary or halo area developer may be carried outeither in the first development stepor inthe second development step,with the understanding that in an image where the secondary developmentstep has been carried `out first followed subsequently by the primarydevelopment, the image in the halo areas is more definitive. Conversely,if the primary development step is first in sequence, then the image inthe thicker casting arcas or normal development areas is moredefinitive.

lt is to be understood that the present invention may be employed forX-rayv examination or similar examination by penetrating radiation ofmedical or industrial test objects. Similarly, there may be used anysuitable radiation sensitive insulating layers charged to eithernegative or positive polarity as may be desired. For purposes ofillustration, the invention has been described in terms of X-rayexamination of industrial castings employing vitreous selenium as theradiation sensitive layer and .with positive polarity charging forsensitization. This is in accord with presently preferred techniques,but it is to be understood thatthe invention is not thus limited in .itsscope. It is similarly to be understood that many variations intechniques and developer materials may be employed within the generalscope and guidance of the present invention.

What is claimed is:

1. In a xeroradiographic inspection method including uniformly charginga photoconductive insulating layer overlying a conductive backingsurface, exposing said layer to a pattern of penetrating radiation toform a charge pattern thereon, and developing said surface by depositingthereon finely divided charged powder particles in accordance with theelectrostatic field associated with said charge pattern, the improvementcomprising again developing said surface, while said powder and chargepattern remain thereon, by depositing thereon in accordance with theelectrostatic field associated with said charge pattern particles ofanother finely divided powder of color different from and chargepolarity opposite to that originally used.

2. A method of developing an electrostatic charge pattern on aninsulating surface comprising first depositing on said surface, inaccordance with the electrostatic field associated with said chargepattern, finely divided particles of a first color .and bearing anelectrostatic charge of a first polarity, and then depositing on saidsurface while said charge pattern and said deposited particles remainthereon and in conformity with the electrostatic field associated withsaid charge pattern finely divided particles of a second colordistinguishable from said first color and bearing an electrostaticcharge of polarity opposite to said first polarity.

3.*A method of developing an electrostatic charge pattern on aninsulating surface overlying a conductive backing comprising firstdepositing on said surface in conformity withthe electrostatic eldassociated with said charge pattern finely divided particles of a firstcolor and bearing an electrostatic charge of a first polarity, and thendepositing on said surface, while said charge pattern and said depositedparticles remain thereon and in conformity with the electrostatic fieldassociated with said charge pattern, finely divided particles of asecond color distinguishable from said rst color and bearing anelectrostatic charge of polarity opposite to said first polarity, saidsecondcolor particles depositing primarily in areas ofthe surface notcovered by said first color particles.

4. A method o-f developing an electrostatic charge pattern on aninsulating surface overlying a conductive backing comprising firstelectrostatically attracting to said surface finely divided particles ofa first color and bearing anelectrostatic charge of a first polarity andthen attracting to said surface while said charge pattern and said firstpolarity powder remains thereon finely divided particles of a secondcolor distinguishable from said first color and bearing an electrostaticcharge of polarity opposite to said first polarity, particles of saidfirst and second colors depositing principally in separate areas.

5. A method of developing an electrostatic charge pattern on aninsulating surface overlying a conductive backing comprising firstexposing said surface to a gaseous suspension of finely divided powderparticles of a rst color and bearing an electrostatic charge of a rstpolarity and then exposing said surface, while said charge pattern andsaid first polarity powder remain thereon, to a gaseous suspension offinely divided powder particles of a second color distinguishable fromsaid first color and bearing an electrostatic charge of polarityopposite to said first polarity, particles `of said second colordepositing primarily in areas of the surface not covered by particles ofsaid rst color.

6. A method of developing a surface carrying a single polarity chargepattern with a corresponding two-polarity electrostatic field patterncomprising first depositing on areas `of said surface having a firstfield polarity finely divided particles of a first color and of oppositepolarity to said rst field polarity and then while said charge patternand said particles remain on said surface, depositing on areas of saidsurface having a second field polarity finely divided particles of asecond color contrasting with said first color and of polarity oppositeto said second field polarity.

References Cited inthe le of this patent Steinhilper July 31, 1956

1. IN A XERORADIOGRAPHIC INSPECTION METHOD INCLUDING UNIFORMLY CHARGINGA PHOTOCONDUCTIVE UNSULATING LAYER OVERLYING A CONDUCTIVE BACKINGSURFACE, EXPOSING SAID LAYER TO A PATTERN OF PENETRATING RADIATION TOFORM A CHARGE PATTERN THEREON, AND DEVELOPING SAID SURFACE BY DEPOSITINGTHEREON FINELY DIVIDED CHARGED POWDER PARTICLES IN ACCORDANCE WITH THEELECTROSTATIC FIELD ASSOCIATED WITH SAID CHARGE PATTERN, THE IMPROVEMENTCOMPRISING AGAIN DEVELOPING SAID SURFACE, WHILE SAID POWDER AND CHANGEPATTERN REMAIN THEREON, BY DEPOSITING THEREON IN ACCORDANCE WITH THEELECTROSTATIC FIELD ASSOCIATED WITH SAID CHARGE PATTERN PARTICLES OFANOTHER FINELY DIVIDED POWDER OF COLOR DIFFERENT FROM AND CHARGEPOLARITY OPPOSITE TO THAT ORIGINALLY USED.